Automotive vehicles having independent suspensions are generally equipped with stabilizer bars to reduce inclination or roll of the vehicle bodies during vehicle maneuvers. The stabilizer bar is usually connected between the suspension arms of the vehicle wheels. When the left and right wheels are in similar positions with respect to the suspensions, the stabilizer bar does not twist so that the suspensions are mutually independent. When one of the left wheel and the right wheel passes over a projection on the road surface, or when the vehicle turns and thus the left wheel and the right wheel assume considerably different positions with respect to the suspensions, the stabilizer bar is twisted. This twisting motion induces a torsional resilient force for affecting the rolling characteristic of the vehicle and of the road-surface-following characteristic of the vehicle wheels.
It is desirable that the torsional resilient force of the stabilizer bars can be adjusted in accordance with running or traveling conditions of the vehicle. Specifically, it is desirable to reduce the torsional rigidity during straight travel of the vehicle and to increase the torsional rigidity during turning of the vehicle. The reduced torsional rigidity enhances the road-surface-following characteristic of the vehicle wheels while the increased torsional rigidity enhances the rolling characteristic of the vehicle.
Certain vehicle active tilt control systems include front and rear stabilizer bars which are adjustable by front and rear hydraulic actuators placed in lieu of the stabilizer bar linkages. The actuators are moveable in first and second opposing directions for adjusting vehicle body roll active roll moment to compensate for vehicle roll.
Current active tilt control systems on the market use flow-controlled pumps and an additional pressure control valve on the main line for controlling the oil pressure which drives the actuators. The flow-controlled pumps provide fluid to a main flow line, and an electronically controlled pressure relief valve of the main flow line controls the fluid pressure. In this method, the entire flow is controlled by the single stage electrically operated pressure relief valve. Typically, flow gain of these valves are limited by design restrictions. The use of such a single stage valve slows system response time, and as a result of this, the system cannot compensate for the roll moment during evasive cornering maneuvers.
Referring to FIGS. 1 and 2, a flow-controlled pump 10 is shown schematically and graphically as used in prior art active tilt control systems. The flow control function within the flow-controlled pump 10 is provided by a flow-control circuit. The flow-controlled pump 10 includes a rotary pump group 12 and a spool valve 14. The rotary pump group 12 draws fluid from the tank 16 and feeds fluid through the spool valve 14 and orifice 18 to the main line 20. The fixed orifice 18 provides a pressure drop as a measure of the flow. This pressure drop is captured by the spool valve 14. At a predetermined pressure difference, the metering orifice of the valve 14 starts relieving oil into the reservoir 16, providing the flat flow characteristic shown in FIG. 2 for different flow rates. This break point A1 or A2 in the flow control characteristic, as shown in FIG. 2, is dependent upon the size of the orifice 18.
In order to provide good flow regulation, the metering orifice of the main flow spool valve has a high flow gradient. However, the buildup of the pressure differential across the main spool 14 is typically a slow process, which results in slow response to flow demand.
Typically, the active tilt control system will have an additional pressure relief valve on the main flow line 20 such that downstream flow is divided again. The first portion of the flow is used for the active tilt control system operation and the second portion of the flow is used for pressure control, i.e., it is returned back to the tank 16. A disadvantage of such a system is that the pump's flow is limited and cannot provide higher instantaneous flow, which results in a slower response time for the active tilt control system. The single stage pressure control valve that controls the pressure in the main line must provide a fast response as well as good flow and pressure characteristics. However, the flow gradient of this valve cannot be as high as the flow gradient of the main spool valve. Even if the single stage pressure control valve has good pressure characteristics, the amount of flow that is flowing through the valve is significant. This means that the actuators do not receive the largest amount of flow when the fast response is needed. Accordingly, flow is wasted.
Therefore, it is desirable to provide an active tilt control system with improved controllability for quicker response during vehicle maneuvers.